It is known that magnetic discs are recorded with concentric circular tracks each divided into sectors, with each sector being provided with a read/write field called a header containing information defining which of the various tracks the sector belongs to and which sector among the various track sectors, and other information. In order to be able to obtain a high track density, and therefore a high disc capacity, and to ensure that data is correctly written to and read from a track, it is necessary to control the position of the head on the track and correcting it if the head is not exactly aligned on the track center.
To this end, in the header field of each sector two short successions of sinusoidal signals, called "servo bursts" of equal amplitude are recorded, one on either side of the track center, over separate arcs of the header field. If, during the course of a subsequent reading, the read head is correctly positioned over the track center, the previously effected recording induces two identical signals in the read head when the two separate arcs of the header field are read. Otherwise, the two signals differ in amplitude and provide an indication of the displacement of the head from the center of the track. This indication is utilized by a control system to correct the position of the head, aligning it on the center of the track.
The known techniques for measuring the displacement of the head from the track center to compare between the two signals read from the servo recording are essentially of two types:
1) Peak detection: the peak, that is, the maximum value of the signals provided by the head upon reading each of the two servo bursts, is memorized and compared with the other. This recognition method has the advantage of requiring relatively simple sampling and memory circuits and of being independent of the timing signals which activate the sampling operation (which must be activated only for reading the two servo bursts, and not when the other data is read). A serious disadvantage of this method is the low immunity to noise and therefore low precision. In fact, a disturbance which causes an increase, even of a single signal peak, contributes its amplitude integrally to the detection of an erroneous amplitude value of the peak. It is to be noted that in the case of signals provided by a read head (generally through a preamplifier) with asymmetrical characteristics, as is the case when the read head is of magnetoresistive type, two identical sampling circuits are necessary, respectively, for the recognition of the maximum positive peak and the maximum negative peak, which is converted into a positive peak with a simple inversion of the connection of the input terminals. The sum of the two measurements provides the peak-to-peak amplitude of the signal.
2) Area detection: This technique is based on the measurement of the area of the rectified signal. Although conceptually very simple, this technique, which involves an integration of the signal over a predetermined time interval, requires complex and expensive circuits. Moreover, the measurement precision depends on the precision with which the integration time interval is defined. The advantage of this technique, which has a high immunity to noise, is that it does not require circuit duplication in the case of asymmetric signals, but this only partly compensates for the above-mentioned disadvantages.
What is desired is a technique which is constructionally simple with a high immunity to noise without being influenced by imprecisions in the timing signal.